EP3589063B1 - Method for performing handover, user equipment, and radio communication system - Google Patents
Method for performing handover, user equipment, and radio communication system Download PDFInfo
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- EP3589063B1 EP3589063B1 EP19191637.8A EP19191637A EP3589063B1 EP 3589063 B1 EP3589063 B1 EP 3589063B1 EP 19191637 A EP19191637 A EP 19191637A EP 3589063 B1 EP3589063 B1 EP 3589063B1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W56/00—Synchronisation arrangements
- H04W56/001—Synchronization between nodes
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W24/00—Supervisory, monitoring or testing arrangements
- H04W24/02—Arrangements for optimising operational condition
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/0005—Control or signalling for completing the hand-off
- H04W36/0011—Control or signalling for completing the hand-off for data sessions of end-to-end connection
- H04W36/0016—Hand-off preparation specially adapted for end-to-end data sessions
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/0005—Control or signalling for completing the hand-off
- H04W36/0083—Determination of parameters used for hand-off, e.g. generation or modification of neighbour cell lists
- H04W36/0085—Hand-off measurements
- H04W36/0088—Scheduling hand-off measurements
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/08—Reselecting an access point
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/24—Reselection being triggered by specific parameters
- H04W36/30—Reselection being triggered by specific parameters by measured or perceived connection quality data
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/24—Reselection being triggered by specific parameters
- H04W36/30—Reselection being triggered by specific parameters by measured or perceived connection quality data
- H04W36/302—Reselection being triggered by specific parameters by measured or perceived connection quality data due to low signal strength
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W72/00—Local resource management
- H04W72/04—Wireless resource allocation
- H04W72/044—Wireless resource allocation based on the type of the allocated resource
- H04W72/0453—Resources in frequency domain, e.g. a carrier in FDMA
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W74/00—Wireless channel access
- H04W74/002—Transmission of channel access control information
- H04W74/004—Transmission of channel access control information in the uplink, i.e. towards network
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W74/00—Wireless channel access
- H04W74/002—Transmission of channel access control information
- H04W74/008—Transmission of channel access control information with additional processing of random access related information at receiving side
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- H—ELECTRICITY
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- H04W74/08—Non-scheduled access, e.g. ALOHA
- H04W74/0833—Random access procedures, e.g. with 4-step access
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W88/00—Devices specially adapted for wireless communication networks, e.g. terminals, base stations or access point devices
- H04W88/08—Access point devices
Definitions
- the present invention relates to a method for performing a handover, a user equipment, and a radio communication system.
- LTE-A Long Term Evolution-Advanced
- BS base station
- eNB evolved Node B
- the bandwidths of frequency bands that are available in LTE are 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, and 20 MHz. Accordingly, if five bands of 20 MHz are aggregated as component carriers, a communication channel of 100 MHz in total can be formed.
- Component carriers that are included in one communication channel in the carrier aggregation are not necessarily contiguous to one another in the frequency direction.
- the mode in which component carriers are arranged contiguous to one another in the frequency direction is called a contiguous mode.
- the mode in which component carriers are arranged not contiguous to one another is called a noncontiguous mode.
- the number of component carriers in an uplink and the number of component carriers in a downlink are not necessarily equal.
- the mode in which the number of component carriers in an uplink and the number of component carriers in a downlink are equal is called a symmetric mode.
- the mode in which the number of component carriers in an uplink and the number of component carriers in a downlink are not equal is called an asymmetric mode.
- any one of frequency division duplex (FDD) and time division duplex (TDD) can be used as duplex operation. Because the direction of a link (uplink or downlink) of each component carrier does not change in time in FDD, FDD is better suited to the carrier aggregation compared to TDD.
- a handover which is a basic technique for achieving the mobility of a user equipment in the cellular communication standard, is one of important subjects in LTE-A.
- a user equipment measures a communication quality over a channel with a serving base station (a currently connected base station) and communication qualities with peripheral base stations and transmits a measurement report containing measurements to the serving base station. Receiving the measurement report, the serving base station determines whether to execute a handover based on the measurements contained in the report.
- a handover is carried out among a source base station (the serving base station before a handover), the user equipment, and a target base station (a serving base station after a handover) in accordance with a prescribed procedure (e.g. cf. Patent Literature 1 below)
- an initial access from a user equipment to a target base station during a handover procedure is performed as a random access over a random access channel which is preset at a prescribed position of communication resources.
- a random access inherently has a possibility of a delay due to signal collision, retry or the like.
- adjustment of transmission timing based on timing of a random access is the overhead of a communication.
- simply repeating random accesses the same number of times as the number of component carriers raises the possibility of degradation of service quality such as an accumulation of delays due to a failure of a random access or an increase in overhead.
- the method for performing a handover, the user equipment, and the radio communication system can minimize the degradation of service quality due to a random access during a handover procedure in a radio communication involving the carrier aggregation.
- FIG. 1 shows a flow of a handover procedure in conformity with LTE in a radio communication not involving the carrier aggregation as an example of a typical handover procedure.
- a user equipment (UE), a source base station (source eNB), a target base station (target eNB), and a mobility management entity (MME) are involved in the handover procedure.
- UE user equipment
- source eNB source base station
- target eNB target base station
- MME mobility management entity
- the user equipment first reports the channel quality of a communication channel between the user equipment and the source base station to the source base station (step S2).
- the channel quality may be reported on a regular basis or when the channel quality falls below a predetermined reference value.
- the user equipment can measure the channel quality of the communication channel with the source base station by receiving a reference signal contained in a downlink channel from the source base station.
- the source base station determines the needs of measurement based on the quality report received from the user equipment and, if measurement is necessary, allocates measurement gaps to the user equipment (step S4).
- the user equipment searches for a downlink channel from a peripheral base station (i.e. performs cell search) during the periods of the allocated measurement gaps (step S12).
- a peripheral base station i.e. performs cell search
- the user equipment can recognize a peripheral base station to search according to a list that is provided in advance from the source base station.
- the user equipment When the user equipment acquires synchronization with a downlink channel, the user equipment performs measurement by using a reference signal contained in the downlink channel (step S14). During this period, the source base station restricts an allocation of data communication related to the user equipment so as to avoid occurrence of data transmission by the user equipment.
- the user equipment Upon completion of the measurement, the user equipment transmits a measurement report containing measurements to the source base station (step S22).
- the measurements contained in the measurement report may be the average value or the central value of measured values over a plurality of times of measurement or the like. Further, the measurements may contain data about a plurality of frequency bands.
- the source base station determines whether or not to execute a handover based on the contents of the measurement report. For example, when the channel quality of another base station in the periphery is higher than the channel quality of the source base station by a predetermined threshold or greater, it can be determined that a handover is necessary. In this case, the source base station determines to carry out a handover procedure with the relevant another base station as a target base station, and transmits a handover request message to the target base station (step S24).
- the target base station determines whether it is possible to accept the user equipment according to the availability of a communication service offered by itself or the like. When it is possible to accept the user equipment, the target base station transmits a handover request confirm message to the source base station (step S26).
- the source base station transmits a handover command to the user equipment (step S28). Then, the user equipment acquires synchronization with the downlink channel of the target base station (step S32). After that, the user equipment makes a random access to the target base station by using a random access channel in a given time slot (step S34). During this period, the source base station forwards data addressed to the user equipment to the target base station (step S36). Then, after success in the random access, the user equipment transmits a handover complete message to the target base station (step S42).
- the target base station requests the MME to perform route update for the user equipment (step S44).
- the user equipment Upon updating the route of user data by the MME, the user equipment becomes able to communicate with another device through a new base station (i.e. the target base station). Then, the target base station transmits acknowledgement to the user equipment (step S46). A series of handover procedure thereby ends.
- Fig. 2 shows a structure of a communication resource in LTE as an example of a structure of a communication resource to which the present invention is applicable.
- the communication resource in LTE is segmented in the time direction into radio frames each having a length of 10 msec.
- One radio frame includes ten sub-frames, and one sub-frame is made up of two 0.5 msec slots.
- the sub-frame is one unit of an allocation of a communication resource to each user equipment in the time direction.
- Such one unit is called a resource block.
- One resource block includes twelve sub-carriers in the frequency direction. Specifically, one resource block has a size of 1 msec with 12 sub-carriers in the timefrequency domain.
- Throughput of data communication increases as a larger number of resource blocks are allocated for data communication on condition of the same bandwidth and time length.
- a part of radio frame with a given frequency band is reserved as a random access channel.
- the random access channel can be used for an access to a base station by a user equipment that has changed from an idle mode to an active mode, for example, in addition to for an initial access to a target base station in a handover procedure as described above.
- Figs. 3A and 3B are sequence charts to further describe general random access procedures.
- Fig. 3A shows a procedure of a contention-based random access.
- the user equipment first transmits a random access preamble to the target base station (step S52).
- the random access preamble is selected from 64 different sequences. Because the 64 sequences are orthogonal to one another, even when the target base station receives different sequences at the same timing with the same frequency, it can separate those sequences. However, when the same sequence is transmitted from different user equipments at the same timing with the same frequency, there is a possibility of collision of random access preambles, which causes a random access to fail.
- the target base station which has received the random access preamble determines a timing adjustment value for each user equipment according to the reception timing (step S54). For example, the random access preamble of a user equipment which is located farther from the target base station arrives at the target base station at the later timing than the random access preamble of a user equipment which is located closer to the target base station. In this case, the target base station allocates a timing adjustment value indicating that a signal should be transmitted earlier to the former (i.e. far) user equipment. Further, the target base station performs scheduling for a connection request of the user equipment. Note that, when a communication resource for a connection request cannot be allocated due to the lack of available communication resources, a random access fails.
- the target base station transmits a random access response to the user equipment (step S56).
- the random access response contains the timing adjustment value determined by the target base station and scheduling information for a connection request by the user equipment.
- the user equipment adjusts the transmission timing of a signal to the target base station by using the timing adjustment value contained in the random access response (step S58).
- the user equipment transmits a connection request to the target base station by using the communication resource which is specified by the scheduling information contained in the random access response (step S60).
- the connection request contains identification information of the user equipment or the like. Note that, for example, there is a possibility that the same random access preamble is transmitted from two or more user equipments in the step S52, and the random access preamble is received normally by the target base station. In such a case, the two or more user equipments which have received a random access response transmitted from the target base station can simultaneously transmit connection requests in the step S60. In this case also, a collision occurs between the connection requests, and a random access can thereby fail.
- the target base station which has received the connection request from the user equipment then transmits acknowledgement to the user equipment (step S62).
- acknowledgement when a communication resource for the acknowledgement cannot be allocated due to the lack of available communication resources, there is a possibility that the acknowledgement is not transmitted.
- the user equipment receives the acknowledgement in the step S62, it recognizes that the random access has succeeded, and a series of random access procedure ends.
- the user equipment does not receive the random access response or the acknowledgement, it determines that the random access has failed and retries the random access after the lapse of a random backoff period.
- Fig. 3B shows a procedure of a contention-free random access.
- the target base station first allocates a random access preamble to the user equipment (step S72).
- the random access preamble for the contention-free random access is selected by the target base station from a predetermined number (e.g. reserved 10) of sequences among 64 orthogonal sequences, for example.
- the user equipment transmits the random access preamble to the target base station (step S74).
- the same random access preamble is used by different user equipments at the same timing with the same frequency, a collision of random access preambles does not occur.
- the target base station which has received the random access preamble determines a timing adjustment value for each user equipment according to the reception timing (step S76). Further, the target base station performs scheduling for a connection request of the user equipment. Note that, when a communication resource for a connection request cannot be allocated due to the lack of available communication resources, a random access fails.
- the target base station transmits a random access response to the user equipment (step S78).
- the random access response contains the timing adjustment value determined by the target base station and scheduling information for a connection request by the user equipment.
- the user equipment adjusts the transmission timing of a signal to the target base station by using the timing adjustment value contained in the random access response (step S80). After that, the user equipment establishes a connection with the target base station in the same manner as the contention-based random access procedure.
- a random access in the random access procedure, a random access can fail due to the collision of signals or the shortage of communication resources. Then, a random access is retried after the lapse of a random backoff period. During this period, a communication by the user equipment delays. Further, the time needed for calculation of a timing adjustment value and adjustment of transmission timing is the overhead of a communication.
- the use of the contention-free random access procedure reduces the possibility of signal collision. However, because the number of random access preambles which are reserved for the contention-free random access is limited, the contention-free random access procedure is not always available in a handover procedure. Therefore, for the purpose of preventing the degradation of service quality, it is effective to perform a random access during a handover procedure in a radio communication involving the carrier aggregation more efficiency as in two embodiments of the present invention which are described in detail below.
- Fig. 4 is a schematic view showing an outline of a radio communication system 1 according to an embodiment of the present invention.
- the radio communication system 1 includes a user equipment 100, a base station 200a and a base station 200b. It is assumed that the base station 200a is a serving base station for the user equipment 100.
- the user equipment 100 is located inside a cell 202a where a radio communication service is provided by the base station 200a.
- the user equipment 100 can perform a data communication with another user equipment (not shown) via the base station 200a over a communication channel formed by aggregating a plurality of component carriers (i.e. by carrier aggregation).
- a handover is required for the user equipment 100.
- the user equipment 100 is located inside a cell 202b where a radio communication service is provided by the base station 200b. Therefore, the base station 200b can be a candidate for a target base station for a handover of the user equipment 100.
- the base station 200a can communicate with the base station 200b through a backhaul link (e.g. X2 interface).
- a backhaul link e.g. X2 interface
- Various kinds of messages in the handover procedure as described with reference to Fig. 1 scheduling information related to the user equipment belonging to each cell or the like, for example, can be transmitted and received between the base station 200a and the base station 200b.
- the base station 200a and the base station 200b can communicate with the MME, which is an upper node, through S1 interface, for example.
- base station 200a and the base station 200b are collectively referred to as a base station 200 by omitting the alphabetical letter at the end of the reference symbol. The same applies to the other elements.
- a first embodiment of the present invention for making a more efficient random access during a handover procedure in a radio communication involving the carrier aggregation is described hereinafter with reference to Figs. 5 to 9B .
- Fig. 5 is a block diagram showing an example of a configuration of the user equipment 100 according to the embodiment.
- the user equipment 100 includes a radio communication unit 110, a signal processing unit 150, a control unit 160, an adjustment value storage unit 162, and a measurement unit 170.
- the radio communication unit 110 performs a radio communication with the base station 200 over a communication channel formed by aggregating a plurality of component carriers with use of the carrier aggregation technology.
- Fig. 6 is a block diagram showing an example of a more detailed configuration of the radio communication unit 110.
- the radio communication unit 110 includes an antenna 112, a switch 114, a low noise amplifier (LNA) 120, a plurality of down-converters 122a to 122c, a plurality of filters 124a to 124c, a plurality of analogue-to-digital converters (ADCs) 126a to 126c, a demodulation unit 128, a modulation unit 130, a plurality of digital-to-analogue converters (DACs) 132a to 132c, a plurality of filters 134a to 134c, a plurality of up-converters 136a to 136c, a combiner 138, and a power amplifier (PA) 140.
- LNA low noise amplifier
- ADCs analogue-to-digital converters
- ADCs analogue-to-digital converters
- DACs digital-to-analogue converter
- the antenna 112 receives a radio signal transmitted from the base station 200 and outputs the received signal to the LNA 120 through the switch 114.
- the LNA 120 amplifies the received signal.
- the down-converter 122a and the filter 124a separate a baseband signal of the first component carrier (CC1) from the received signal amplified by the LNA 120. Then, the separated baseband signal is converted to a digital signal by the ADC 126a and output to the demodulation unit 128.
- the down-converter 122b and the filter 124b separate a baseband signal of the second component carrier (CC2) from the received signal amplified by the LNA 120.
- the separated baseband signal is converted to a digital signal by the ADC 126b and output to the demodulation unit 128.
- the down-converter 122c and the filter 124c separate a baseband signal of the third component carrier (CC3) from the received signal amplified by the LNA 120.
- the separated baseband signal is converted to a digital signal by the ADC 126c and output to the demodulation unit 128.
- the demodulation unit 128 generates a data signal by demodulating the baseband signals of the respective component carriers and outputs the data signal to the signal processing unit 150.
- the modulation unit 130 modulates the data signal and generates baseband signals of the respective component carriers.
- the baseband signal of the first component carrier (CC1) is converted to an analog signal by the DAC 132a.
- a frequency component corresponding to the first component carrier in a transmission signal is generated from the analog signal by the filter 134a and the up-converter 136a.
- the baseband signal of the second component carrier (CC2) is converted to an analog signal by the DAC 132b.
- a frequency component corresponding to the second component carrier in the transmission signal is generated from the analog signal by the filter 134b and the up-converter 136b.
- the baseband signal of the third component carrier (CC3) is converted to an analog signal by the DAC 132c. Then, a frequency component corresponding to the third component carrier in the transmission signal is generated from the analog signal by the filter 134c and the up-converter 136c. After that, the generated frequency components corresponding to the three component carriers are combined by the combiner 138, and the transmission signal is formed.
- the PA 140 amplifiers the transmission signal and outputs the transmission signal to the antenna 112 through the switch 114. Then, the antenna 112 transmits the transmission signal as a radio signal to the base station 200.
- the number of component carriers handled by the radio communication unit 110 may be two, or four or more.
- the radio communication unit 110 may process the signals of the respective component carriers in the digital region.
- a digital signal converted by one ADC is separated into the signals of the respective component carriers by a digital filter.
- the signal is converted into an analog signal by one DAC.
- the load of the ADC and the DAC is generally smaller when processing the signals of the respective component carriers in the analog region.
- a sampling frequency for AD/DA conversion is higher, and the load of the ADC and the DAC can thereby increase.
- the signal processing unit 150 performs signal processing such as deinterleaving, decoding or error correction on the demodulated data signal that is input from the radio communication unit 110. Then, the signal processing unit 150 outputs the processed data signal to an upper layer. Further, the signal processing unit 150 performs signal processing such as encoding or interleaving on the data signal that is input from the upper layer. Then, the signal processing unit 150 outputs the processed data signal to the radio communication unit 110.
- the control unit 160 controls the overall functions of the user equipment 100 by using a processing device such as a central processing unit (CPU) or a digital signal processor (DSP). For example, the control unit 160 controls the timing of data communication by the radio communication unit 110 according to scheduling information that is received from the base station 200 by the radio communication unit 110. At this time, the control unit 160 adjusts the timing of data transmission from the radio communication unit 110 by using a timing adjustment value which is notified from the base station 200 in a random access procedure. Further, the control unit 160 controls the measurement unit 170 to measure the channel quality by using a reference signal from the base station 200, which is a serving base station, and transmits the channel quality report to the base station 200 through the radio communication unit 110. Further, the control unit 160 controls the measurement unit 170 to execute measurement during the periods of measurement gaps which are allocated by the base station 200.
- a processing device such as a central processing unit (CPU) or a digital signal processor (DSP).
- the control unit 160 controls the timing of data communication by the radio communication unit 110 according
- the control unit 160 determines whether a distance in the frequency direction between the two or more component carriers is smaller than a specific threshold. When the distance in the frequency direction is smaller than a specific threshold, the control unit 160 skips a random access to the target base station for at least one of the two or more component carriers. Specifically, when first and second component carriers exist in close proximity in the frequency direction, the control unit 160 makes a random access for the first component carrier and skips a random access for the second component carrier.
- the specific threshold may be the maximum value of a difference between two frequencies (e.g.
- the user equipment 100 can carry out a handover by appropriately adjusting the transmission timing for the component carrier.
- a transmission timing difference depends on a distance between each user equipment and a base station.
- a timing adjustment is typically used between user equipment and a base station for purposes of synchronization.
- propagation characteristics include, but not limited to, characteristics of reflection, diffraction, scattering and propagation.
- the control unit 160 may skip a random access for at least one of the component carriers. For example, it is assumed that a distance in the frequency direction between the component carrier CC1 and the component carrier CC2 among the component carriers CC1 to CC3 is smaller than a specific threshold, and a distance between the component carrier CC1, CC2 and the component carrier CC3 is larger than the specific threshold. In this case, the control unit 160 may make a random access to the target base station for the component carriers CC1 and CC2 in one time and further make a random access to the target base station for the component carrier CC3.
- the control unit 160 In the case of skipping a random access for any of the component carriers, the control unit 160 first executes a random access to the target base station for another component carrier which exists in close proximity to the relevant component carrier in the frequency direction. Then, the control unit 160 stores the timing adjustment value which is notified from the target base station in the wake of the random access into the adjustment value storage unit 162. After that, the control unit 160 adjusts the transmission timing for the component carrier for which a random access is skipped by using the timing adjustment value stored in the adjustment value storage unit 162. Then, the control unit 160 makes an access to the target base station over the relevant component carrier at the adjusted transmission timing (the access in this case is not a random access).
- the adjustment value storage unit 162 stores a timing adjustment value for one or more component carriers which is input from the control unit 160 by using a storage medium such as a hard disk or semiconductor memory. Then, in response to a command from the control unit 160, the adjustment value storage unit 162 outputs the stored timing adjustment value for use in adjustment of the transmission timing for another component carrier.
- the measurement unit 170 measures the channel quality for each of the component carriers by using a reference signal from the base station 200 according to control from the control unit 160, for example. Further, the measurement unit 170 executes measurement for a handover with respect to each of the component carriers by using the measurement gaps which are allocated by the base station 200. A result of the measurement executed by the measurement unit 170 is converted to a predetermined format for a measurement report by the control unit 160 and transmitted to the base station 200 through the radio communication unit 110. After that, the base station 200 determines, based on the measurement report, whether a handover should be executed or not for the user equipment 100.
- Fig. 7 is a block diagram showing an example of a configuration of the base station 200 according to the embodiment.
- the base station 200 includes a radio communication unit 210, an interface unit 250, a component carrier (CC) management unit 260, and a control unit 280.
- CC component carrier
- a specific configuration of the radio communication unit 210 may be similar to the configuration of the radio communication unit 110 of the user equipment 100 which is described above with reference to Fig. 6 , although the number of component carriers to be supported, the requirements of processing performance or the like are different.
- the radio communication unit 210 performs a radio communication with the user equipment over a communication channel which is formed by aggregating a plurality of component carriers with use of the carrier aggregation technology.
- the interface unit 250 mediates a communication between the radio communication unit 210 or the control unit 280 and an upper node through the S1 interface illustrated in Fig. 4 , for example. Further, the interface unit 250 mediates a communication between the radio communication unit 210 or the control unit 280 and another base station through the X2 interface illustrated in Fig. 4 , for example.
- the CC management unit 260 holds data that indicates which component carrier each user equipment is using for communication with respect to each of the user equipments belonging to the cell of the base station 200. Such data can be updated by the control unit 280 when an additional user equipment joins the cell of the base station 200 or when the existing user equipment changes its component carriers. Thus, the control unit 280 can recognize which component carrier the user equipment 100 is using by referring to the data held by the CC management unit 260.
- the control unit 280 controls the overall functions of the base station 200 by using a processing device such as a CPU or a DSP. For example, the control unit 280 executes an allocation of communication resources for data communication, i.e. scheduling, for the user equipment 100 and other user equipments. Then, the control unit 280 delivers scheduling information over a broadcast channel in a given sub-frame. Further, when the base station 200 is a target base station of a handover of a radio communication involving the carrier aggregation, the control unit 280 allocates two or more component carriers to a new communication channel. At this time, when two or more component carriers which are adjacent in the frequency direction are allocable, the control unit 280 allocates the two or more component carriers to the new communication channel. Further, when two or more component carriers whose distance in the frequency direction is smaller than the above-described specific threshold are allocable, the control unit 280 allocates the two or more component carriers to the new communication channel.
- a processing device such as a CPU or a DSP.
- Fig. 8 is an explanatory view to describe an allocation of component carriers by a target base station according to the embodiment.
- Fig. 8 shows a step-by-step handover including four stages in the case where a communication channel is composed of three component carriers CC1 to CC3. The first stage is before a handover. The second stage is after a handover of the component carrier CC1. The third stage is after a handover of the component carriers CC1 and CC2. The fourth stage is after a handover of all the component carriers.
- the component carriers CC1 to CC3 are connected with a source base station.
- the positions of the component carriers CC1 to CC3 in the frequency direction may be any positions.
- the frequency band to which the component carrier CC1 is allocated in the target base station may be the same frequency band as the frequency band in the source base station or a different frequency band from the frequency band in the source base station.
- the control unit 280 of the target base station which has received a handover request for the component carrier CC2 determines whether the component carriers CC1 and CC2 can be allocated so that they are adjacent in the frequency direction. It is assumed in the example of Fig. 8 that the component carriers CC1 and CC2 can be allocated so that they are adjacent in the frequency direction. Thus, the control unit 28 allocates the component carriers CC1 and CC2 so that they are adjacent in the frequency direction (third stage).
- control unit 280 of the target base station which has received a handover request for the component carrier CC3 determines whether the component carrier CC1 or CC2 and the component carrier CC3 can be allocated so that they are adjacent in the frequency direction. It is assumed in the example of Fig. 8 that those component carriers are not allocable to be adjacent in the frequency direction. Then, the control unit 28 determines whether the component carrier CC3 can be allocated to a new communication channel so that a distance from the component carrier CC1 or CC2 in the frequency direction is smaller than a threshold F TH .
- the threshold F TH indicates the above-described specified threshold which can correspond to the maximum value of a difference between two frequencies whose transmission timing can be adjusted appropriately based on one timing adjustment value.
- the component carrier CC3 can be allocated so that the distances from the component carriers CC1 and CC2 are smaller than the threshold F TH .
- the control unit 280 allocates the component carrier CC3 to the position where the distances from the component carriers CC1 and CC2 are smaller than the threshold F TH (fourth stage).
- Fig. 8 shows the case where, after a handover for one component carrier is completed, another component carrier is allocated to the new communication channel.
- the allocation of another component carrier to a new communication channel may be performed before completion of a handover for one component carrier.
- the frequency band of the component carrier is changed at the time of an access to the target base station by the user equipment 100.
- a base station to which the component carrier is connected may be changed from the source base station to the target base station after the frequency band has changed in the source base station according to the above-described conditions related to the position in the frequency direction.
- control unit 280 controls the base station 200 to operate in the same manner as the source base station or the target base station in the handover procedure which is descried with reference to Fig. 1 .
- a flow of a handover procedure according to the embodiment is described hereinafter with reference to Figs. 9A and 9B .
- a handover procedure is performed among the user equipment 100, the base station 200a serving as a source base station, and the base station 200b serving as a target base station.
- the user equipment 100 performs a radio communication by using two component carriers.
- explanation is omitted because there is no significant difference.
- the user equipment 100 first transmits a measurement report for the component carrier CC1, for example, to the base station 200a (step S122).
- the base station 200a determines the necessity of a handover based on the measurement report. For example, when a channel quality between the user equipment 100 and the base station 200b is better than a channel quality between the user equipment 100 and the base station 200a by a predetermined threshold or greater, it can be determined that a handover is necessary.
- the base station 200a transmits a handover request message for the component carrier CC1 to the base station 200b (step S124).
- the base station 200b allocates the component carrier CC1 to any frequency band for a new communication channel with the user equipment 100 according to the availability of a communication service. Then, the base station 200b transmits a handover request confirm message to the base station 200a (step S126). Receiving the handover request confirm message, the base station 200a transmits a handover command for the component carrier CC1 to the user equipment 100 (step S128).
- the user equipment 100 receives the handover command, the user equipment 100 first acquires synchronization with the downlink channel of the component carrier CC1 of the base station 200b (step S132). Then, the user equipment 100 makes a random access to the base station 200b by using a random access channel of the component carrier CC1 (step S134). During this period, the control unit 160 of the user equipment 100 stores the timing adjustment value for the component carrier CC1 which is notified from the base station 200b into the adjustment value storage unit 162. During the downlink synchronization and the random access, the base station 200a forwards data addressed to the user equipment 100 to the base station 200b (step S136).
- the user equipment 100 transmits a handover complete message for the component carrier CC1 to the base station 200b (step S142).
- the base station 200b requests the MME to perform route update for the component carrier CC1 of the user equipment 100 (step S144).
- the user equipment 100 becomes able to communicate with another device through a new base station (i.e. the base station 200b).
- the request for route update may be performed with respect to each component carrier, or performed only once through a plurality of component carriers.
- the base station 200b transmits acknowledgement for the handover complete message to the user equipment 100 (step S146).
- the user equipment 100 transmits a measurement report for the component carrier CC2 to the base station 200a (step S152).
- the base station 200a receives the measurement report, the base station 200a transmits a handover request message for the component carrier CC2 to the base station 200b (step S154).
- the base station 200b allocates the component carrier CC2 to any frequency band for the new communication channel with the user equipment 100 according to the availability of a communication service.
- the frequency band to which the component carrier CC2 is allocated is the frequency band which is adjacent to the component carrier CC1 or the frequency band in which the distance from the component carrier CC1 in the frequency direction is smaller than the threshold F TH .
- the base station 200b transmits a handover request confirm message to the base station 200a (step S156).
- the base station 200a transmits a handover command for the component carrier CC2 to the user equipment 100 (step S158).
- the control unit 160 reads the timing adjustment value for the component carrier CC1 which is stored in the adjustment value storage unit 162. The control unit 160 then adjusts the access timing to the base station 200b for the component carrier CC2 by using the timing adjustment value which is read from the adjustment value storage unit 162 (step S164). During this period, the base station 200a forwards data addressed to the user equipment 100 to the base station 200b (step S166).
- the user equipment 100 then transmits a handover complete message for the component carrier CC2 to the base station 200b (step S172).
- the base station 200b requests the MME to perform route update for the component carrier CC2 of the user equipment 100 (step S174).
- the base station 200b transmits acknowledgement for the handover complete message to the user equipment 100 (step S176).
- the first embodiment of the present invention is described above with reference to Figs. 5 to 9B .
- the user equipment 100 skips a random access to the target base station for at least one of the two or more component carriers. Therefore, an accumulation of delays corresponding to the number of component carriers due to a failure of a random access does not occur, and the degradation of service quality caused by a random access during a handover procedure is suppressed.
- the user equipment 100 adjusts the transmission timing for the component carrier for which a random access has been skipped by using the timing adjustment value notified from the target base station for the component carrier for which a random access has been made.
- a communication can be thereby performed at appropriate timing also in the component carrier for which a random access has been skipped. Further, because it is not necessary in the target base station to redundantly determine the timing adjustment value for the component carrier, the overhead of a communication is reduced.
- the base station 200 serving as a target base station allocates the two or more component carriers to a new communication channel with the user equipment 100. Further, when two or more component carriers whose distance in the frequency direction is smaller than a specific threshold are allocable, the base station 200 allocates the two or more component carriers to a new communication channel.
- the opportunity of skipping a random access by the user equipment 100 during a handover procedure thereby increases, and it is possible to more effectively obtain the above-described advantages such as prevention of an accumulation of delays due to a failure of a random access or reduction of overhead. Note that the positions in the frequency direction of those component carriers may be varied after the end of a series of handover procedure as a matter of course.
- a second embodiment of the present invention for making a more efficient random access during a handover procedure in a radio communication involving the carrier aggregation is described hereinafter with reference to Figs. 10 to 12B .
- Fig. 10 is a block diagram showing an example of a configuration of the user equipment 300 according to the embodiment.
- the user equipment 300 includes a radio communication unit 110, a signal processing unit 150, a control unit 360, and a measurement unit 170.
- the control unit 360 controls the overall functions of the user equipment 300 by using a processing device such as a CPU or a DSP. For example, the control unit 360 controls the timing of data communication by the radio communication unit 110 according to scheduling information that is received from the base station 400 by the radio communication unit 110. Further, the control unit 360 controls the measurement unit 170 to measure the channel quality by using a reference signal from the base station 400, which is a serving base station, and transmits the channel quality report to the base station 400 through the radio communication unit 110. Further, the control unit 360 controls the measurement unit 170 to execute measurement during the periods of measurement gaps which are allocated by the base station 400.
- a processing device such as a CPU or a DSP.
- the control unit 360 controls the timing of data communication by the radio communication unit 110 according to scheduling information that is received from the base station 400 by the radio communication unit 110. Further, the control unit 360 controls the measurement unit 170 to measure the channel quality by using a reference signal from the base station 400, which is a serving base station, and transmits the channel quality report to the
- the control unit 360 controls the radio communication unit 110 to make a random access to the base station 400.
- a new communication channel with the base station 400 is established.
- the control unit 360 controls the radio communication unit 110 to notify the base station 400 of the timing of a random access for one or more other (remaining) component carriers to constitute the communication channel over the established new communication channel.
- the base station 400 as a target base station can thereby reserve communication resources necessary in the random access procedure by the user equipment 300 in advance based on the notified timing.
- Fig. 11 is a block diagram showing an example of a configuration of the base station 400 according to the embodiment.
- the base station 400 includes a radio communication unit 210, an interface unit 250, a CC management unit 260, and a control unit 480.
- the control unit 480 controls the overall functions of the base station 400 by using a processing device such as a CPU or a DSP. For example, the control unit 480 executes an allocation of communication resources for data communication for the user equipment 300 and other user equipments. Then, the control unit 480 delivers scheduling information over a broadcast channel in a given sub-frame. Further, when the base station 400 is a target base station of a handover by the user equipment 300, the control unit 480 sequentially allocates two or more component carriers to a new communication channel with the user equipment 300. At this time, over the new communication channel which is established by a random access for one or more component carriers, the timing of a random access for one or more other component carriers to constitute the communication channel can be notified from the user equipment 300 as described above.
- a processing device such as a CPU or a DSP.
- the control unit 480 executes an allocation of communication resources for data communication for the user equipment 300 and other user equipments. Then, the control unit 480 delivers scheduling information over a broadcast channel in a
- the control unit 480 reserves communication resources for the user equipment 300 according to the notified timing so that the subsequent random access for the one or more other component carriers succeeds.
- the communication resources reserved thereby include resources of an uplink for a connection request from the user equipment 300, resources of a downlink for acknowledgement to the connection request or the like.
- control unit 480 controls the base station 400 to operate in the same manner as the source base station or the target base station in the handover procedure which is descried with reference to Fig. 1 .
- a flow of a handover procedure according to the embodiment is described hereinafter with reference to Figs. 12A and 12B .
- a handover procedure is performed among the user equipment 300, the base station 400a serving as a source base station, and the base station 400b serving as a target base station.
- the user equipment 300 performs a radio communication by using two component carriers.
- explanation is omitted because there is no significant difference.
- the user equipment 300 first transmits a measurement report for the component carrier CC1, for example, to the base station 400a (step S222).
- the base station 400a determines that a handover is necessary
- the base station 400a transmits a handover request message for the component carrier CC1 to the base station 400b (step S224).
- the base station 400b allocates the component carrier CC1 to any frequency band for a new communication channel with the user equipment 300 according to the availability of a communication service.
- the base station 400b transmits a handover request confirm message to the base station 400a (step S226).
- the base station 400a transmits a handover command for the component carrier CC1 to the user equipment 300 (step S228).
- the user equipment 300 receives the handover command, the user equipment 300 first acquires synchronization with the downlink channel of the component carrier CC1 of the base station 400b (step S232). Then, the user equipment 300 makes a random access to the base station 400b by using a random access channel of the component carrier CC1 (step S234). When the random access is success, a new communication channel between the user equipment 300 and the base station 400b is established. During the downlink synchronization and the random access, the base station 400a forwards data addressed to the user equipment 300 to the base station 400b (step S236).
- the user equipment 300 transmits a handover complete message for the component carrier CC1 to the base station 400b (step S242).
- the base station 400b requests the MME to perform route update for the component carrier CC1 of the user equipment 300 (step S244).
- the base station 400b transmits acknowledgement for the handover complete message to the user equipment 300 (step S246).
- the user equipment 300 transmits a measurement report for the component carrier CC2 to the base station 400a (step S252).
- the base station 400a receives the measurement report, the base station 400a transmits a handover request message for the component carrier CC2 to the base station 400b (step S254).
- the base station 400b allocates the component carrier CC2 to any frequency band for the new communication channel with the user equipment 300 according to the availability of a communication service.
- the frequency band to which the component carrier CC2 is allocated may be any frequency band.
- the base station 400b transmits a handover request confirm message to the base station 400a (step S256).
- the base station 400a transmits a handover command for the component carrier CC2 to the user equipment 300 (step S258).
- the user equipment 300 notifies the timing of a random access (RA) for the component carrier CC2 to the base station 400b over the established new communication channel (which has the component carrier CC1) (step S260).
- the base station 400b reserves communication resources according to the notified timing so that a random access for the component carrier CC2 by the user equipment 300 does not fail due to the shortage of communication resources (step S261).
- the user equipment 300 acquires synchronization with the downlink channel of the component carrier CC2 of the base station 400b (step S262). Then, the user equipment 300 makes a random access to the base station 400b by using a random access channel of the component carrier CC2 at the timing notified to the base station 400b in the step S260 (step S264). Such a random access is likely to succeed unless signal collision occurs because communication resources for a connection request from the user equipment 300 and acknowledgement for the request are reserved by the base station 400b.
- the base station 400a forwards data addressed to the user equipment 300 to the base station 400b (step S266).
- the user equipment 300 then transmits a handover complete message for the component carrier CC2 to the base station 400b (step S272).
- the base station 400b requests the MME to perform route update for the component carrier CC2 of the user equipment 300 (step S274).
- the base station 400b transmits acknowledgement for the handover complete message to the user equipment 300 (step S276).
- the second embodiment of the present invention is described above with reference to Figs. 10 to 12B .
- the timing of a random access for a component carrier for which a handover has not been completed is notified from the user equipment 300 to the base station 400 over a component carrier for which a handover has been completed.
- the base station 400 can thereby reserve communication resources so that the subsequent random access for another component carrier from the user equipment 300 does not fail due to the shortage of communication resources.
- the possibility of a failure of a random access is reduced, and the degradation of service quality due to an accumulation of delays is suppressed.
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Description
- The present invention relates to a method for performing a handover, a user equipment, and a radio communication system.
- In Long Term Evolution-Advanced (LTE-A), which is the next-generation cellular communication standard that is discussed in Third Generation Partnership Project (3GPP), introduction of technology called carrier aggregation (CA) has been studied. The carrier aggregation is technology that forms a communication channel between a user equipment (UE) and a base station (BS, or evolved Node B (eNB)) by aggregating a plurality of frequency bands that are supported in LTE, for example, and thereby improves communication throughput. Each frequency band included in one communication channel by the carrier aggregation is called a component carrier (CC). The bandwidths of frequency bands that are available in LTE are 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, and 20 MHz. Accordingly, if five bands of 20 MHz are aggregated as component carriers, a communication channel of 100 MHz in total can be formed.
- Component carriers that are included in one communication channel in the carrier aggregation are not necessarily contiguous to one another in the frequency direction. The mode in which component carriers are arranged contiguous to one another in the frequency direction is called a contiguous mode. On the other hand, the mode in which component carriers are arranged not contiguous to one another is called a noncontiguous mode.
- Further, in the carrier aggregation, the number of component carriers in an uplink and the number of component carriers in a downlink are not necessarily equal. The mode in which the number of component carriers in an uplink and the number of component carriers in a downlink are equal is called a symmetric mode. On the other hand, the mode in which the number of component carriers in an uplink and the number of component carriers in a downlink are not equal is called an asymmetric mode. For example, in the case of using two component carriers in an uplink and three component carriers in a downlink, it is asymmetric carrier aggregation.
- Further, in LTE, any one of frequency division duplex (FDD) and time division duplex (TDD) can be used as duplex operation. Because the direction of a link (uplink or downlink) of each component carrier does not change in time in FDD, FDD is better suited to the carrier aggregation compared to TDD.
- A handover, which is a basic technique for achieving the mobility of a user equipment in the cellular communication standard, is one of important subjects in LTE-A. In LTE, a user equipment measures a communication quality over a channel with a serving base station (a currently connected base station) and communication qualities with peripheral base stations and transmits a measurement report containing measurements to the serving base station. Receiving the measurement report, the serving base station determines whether to execute a handover based on the measurements contained in the report. Then, if it is determined that a handover is to be executed, a handover is carried out among a source base station (the serving base station before a handover), the user equipment, and a target base station (a serving base station after a handover) in accordance with a prescribed procedure (e.g. cf.
Patent Literature 1 below) - [PTL 1]
Japanese Unexamined Patent Application Publication No. 2009-232293 - NOKIA CORPORATION ET AL: "RACH and carrier aggregation", 3GPP DRAFT; R2-096844_RACH AND CARRIER AGGREGATION, 3RD GENERATION PARTNERSHIP PROJECT (3GPP), MOBILE COMPETENCE CENTRE; 20091109, 9 November 2009, XP050391275
- However, no case has been reported where active consideration is given to how to carry out a handover procedure in a radio communication involving the carrier aggregation.
- For example, an initial access from a user equipment to a target base station during a handover procedure is performed as a random access over a random access channel which is preset at a prescribed position of communication resources. However, a random access inherently has a possibility of a delay due to signal collision, retry or the like. Further, adjustment of transmission timing based on timing of a random access is the overhead of a communication. Thus, in a radio communication involving the carrier aggregation, simply repeating random accesses the same number of times as the number of component carriers raises the possibility of degradation of service quality such as an accumulation of delays due to a failure of a random access or an increase in overhead.
- In light of the foregoing, it is desirable to provide a novel and improved method for performing a handover, user equipment, and radio communication system that can minimize the degradation of service quality due to a random access during a handover procedure in a radio communication involving the carrier aggregation.
- There is provided a target base station of a handover procedure as well as a method for a target base station of a handover procedure according to independent claims.
- The method for performing a handover, the user equipment, and the radio communication system according to the embodiments of the present invention can minimize the degradation of service quality due to a random access during a handover procedure in a radio communication involving the carrier aggregation.
-
- [
Fig. 1 ]
Fig. 1 is a sequence chart to describe a flow of a typical handover procedure. - [
Fig. 2 ]
Fig. 2 is an explanatory view to describe an example of a structure of a communication resource. - [
Fig. 3A ]
Fig. 3A is a sequence chart to describe a procedure of a contention-based random access. - [
Fig. 3B ]
Fig. 3B is a sequence chart to describe a procedure of a contention-free random access. - [
Fig. 4 ]
Fig. 4 is a schematic view showing an outline of a radio communication system according to an embodiment. - [
Fig. 5 ]
Fig. 5 is a block diagram showing an example of a configuration of a user equipment according to a first embodiment. - [
Fig. 6 ]
Fig. 6 is a block diagram showing an example of a detailed configuration of a radio communication unit according to the first embodiment. - [
Fig. 7 ]
Fig. 7 is a block diagram showing an example of a configuration of a base station according to the first embodiment. - [
Fig. 8 ]
Fig. 8 is an explanatory view to describe an allocation of component carriers by a target base station according to the first embodiment. - [
Fig. 9A ]
Fig. 9A is the first half of a sequence chart showing an example of a flow of a handover procedure according to the first embodiment. - [
Fig. 9B ]
Fig. 9B is the second half of a sequence chart showing an example of a flow of a handover procedure according to the first embodiment. - [
Fig. 10 ]
Fig. 10 is a block diagram showing an example of a configuration of a user equipment according to a second embodiment. - [
Fig. 11 ]
Fig. 11 is a block diagram showing an example of a configuration of a base station according to the second embodiment. - [
Fig. 12A ]
Fig. 12A is the first half of a sequence chart showing an example of a flow of a handover procedure according to the second embodiment. - [
Fig. 12B ]
Fig. 12B is the second half of a sequence chart showing an example of a flow of a handover procedure according to the second embodiment. - Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the appended drawings. Note that, in this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted.
- Preferred embodiments of the present invention will be described hereinafter in the following order.
- 1. Description of Related Art
- 1-1. Handover Procedure
- 1-2. Structure of Communication Resource
- 1-3. Description of Issue Related to Random Access
- 2. Outline of Radio Communication System
- 3. Description of First Embodiment
- 3-1. Exemplary Configuration of User Equipment
- 3-2. Exemplary Configuration of Base Station
- 3-3. Flow of Process
- 3-4. Summary of First Embodiment
- 4. Description of Second Embodiment
- 4-1. Exemplary Configuration of User Equipment
- 4-2. Exemplary Configuration of Base Station
- 4-3. Flow of Process
- 4-4. Summary of Second Embodiment
- A technique related to the present invention is described hereinafter with reference to
Figs. 1 to 3B .Fig. 1 shows a flow of a handover procedure in conformity with LTE in a radio communication not involving the carrier aggregation as an example of a typical handover procedure. In this example, a user equipment (UE), a source base station (source eNB), a target base station (target eNB), and a mobility management entity (MME) are involved in the handover procedure. - As a preliminary step toward a handover, the user equipment first reports the channel quality of a communication channel between the user equipment and the source base station to the source base station (step S2). The channel quality may be reported on a regular basis or when the channel quality falls below a predetermined reference value. The user equipment can measure the channel quality of the communication channel with the source base station by receiving a reference signal contained in a downlink channel from the source base station.
- Then, the source base station determines the needs of measurement based on the quality report received from the user equipment and, if measurement is necessary, allocates measurement gaps to the user equipment (step S4).
- Then, the user equipment searches for a downlink channel from a peripheral base station (i.e. performs cell search) during the periods of the allocated measurement gaps (step S12). Note that the user equipment can recognize a peripheral base station to search according to a list that is provided in advance from the source base station.
- When the user equipment acquires synchronization with a downlink channel, the user equipment performs measurement by using a reference signal contained in the downlink channel (step S14). During this period, the source base station restricts an allocation of data communication related to the user equipment so as to avoid occurrence of data transmission by the user equipment.
- Upon completion of the measurement, the user equipment transmits a measurement report containing measurements to the source base station (step S22). The measurements contained in the measurement report may be the average value or the central value of measured values over a plurality of times of measurement or the like. Further, the measurements may contain data about a plurality of frequency bands.
- Receiving the measurement report, the source base station determines whether or not to execute a handover based on the contents of the measurement report. For example, when the channel quality of another base station in the periphery is higher than the channel quality of the source base station by a predetermined threshold or greater, it can be determined that a handover is necessary. In this case, the source base station determines to carry out a handover procedure with the relevant another base station as a target base station, and transmits a handover request message to the target base station (step S24).
- Receiving the handover request message, the target base station determines whether it is possible to accept the user equipment according to the availability of a communication service offered by itself or the like. When it is possible to accept the user equipment, the target base station transmits a handover request confirm message to the source base station (step S26).
- Receiving the handover request confirm message, the source base station transmits a handover command to the user equipment (step S28). Then, the user equipment acquires synchronization with the downlink channel of the target base station (step S32). After that, the user equipment makes a random access to the target base station by using a random access channel in a given time slot (step S34). During this period, the source base station forwards data addressed to the user equipment to the target base station (step S36). Then, after success in the random access, the user equipment transmits a handover complete message to the target base station (step S42).
- Receiving the handover complete message, the target base station requests the MME to perform route update for the user equipment (step S44). Upon updating the route of user data by the MME, the user equipment becomes able to communicate with another device through a new base station (i.e. the target base station). Then, the target base station transmits acknowledgement to the user equipment (step S46). A series of handover procedure thereby ends.
-
Fig. 2 shows a structure of a communication resource in LTE as an example of a structure of a communication resource to which the present invention is applicable. Referring toFig. 2 , the communication resource in LTE is segmented in the time direction into radio frames each having a length of 10 msec. One radio frame includes ten sub-frames, and one sub-frame is made up of two 0.5 msec slots. In LTE, the sub-frame is one unit of an allocation of a communication resource to each user equipment in the time direction. Such one unit is called a resource block. One resource block includes twelve sub-carriers in the frequency direction. Specifically, one resource block has a size of 1 msec with 12 sub-carriers in the timefrequency domain. Throughput of data communication increases as a larger number of resource blocks are allocated for data communication on condition of the same bandwidth and time length. Further, in such a structure of a communication resource, a part of radio frame with a given frequency band is reserved as a random access channel. The random access channel can be used for an access to a base station by a user equipment that has changed from an idle mode to an active mode, for example, in addition to for an initial access to a target base station in a handover procedure as described above. -
Figs. 3A and3B are sequence charts to further describe general random access procedures.Fig. 3A shows a procedure of a contention-based random access. - Referring to
Fig. 3A , the user equipment first transmits a random access preamble to the target base station (step S52). The random access preamble is selected from 64 different sequences. Because the 64 sequences are orthogonal to one another, even when the target base station receives different sequences at the same timing with the same frequency, it can separate those sequences. However, when the same sequence is transmitted from different user equipments at the same timing with the same frequency, there is a possibility of collision of random access preambles, which causes a random access to fail. - Next, the target base station which has received the random access preamble determines a timing adjustment value for each user equipment according to the reception timing (step S54). For example, the random access preamble of a user equipment which is located farther from the target base station arrives at the target base station at the later timing than the random access preamble of a user equipment which is located closer to the target base station. In this case, the target base station allocates a timing adjustment value indicating that a signal should be transmitted earlier to the former (i.e. far) user equipment. Further, the target base station performs scheduling for a connection request of the user equipment. Note that, when a communication resource for a connection request cannot be allocated due to the lack of available communication resources, a random access fails.
- Then, the target base station transmits a random access response to the user equipment (step S56). The random access response contains the timing adjustment value determined by the target base station and scheduling information for a connection request by the user equipment. Receiving the random access response, the user equipment adjusts the transmission timing of a signal to the target base station by using the timing adjustment value contained in the random access response (step S58).
- Then, the user equipment transmits a connection request to the target base station by using the communication resource which is specified by the scheduling information contained in the random access response (step S60). The connection request contains identification information of the user equipment or the like. Note that, for example, there is a possibility that the same random access preamble is transmitted from two or more user equipments in the step S52, and the random access preamble is received normally by the target base station. In such a case, the two or more user equipments which have received a random access response transmitted from the target base station can simultaneously transmit connection requests in the step S60. In this case also, a collision occurs between the connection requests, and a random access can thereby fail.
- The target base station which has received the connection request from the user equipment then transmits acknowledgement to the user equipment (step S62). Note that, when a communication resource for the acknowledgement cannot be allocated due to the lack of available communication resources, there is a possibility that the acknowledgement is not transmitted. When the user equipment receives the acknowledgement in the step S62, it recognizes that the random access has succeeded, and a series of random access procedure ends. On the other hand, when the user equipment does not receive the random access response or the acknowledgement, it determines that the random access has failed and retries the random access after the lapse of a random backoff period.
-
Fig. 3B shows a procedure of a contention-free random access. Referring toFig. 3B , the target base station first allocates a random access preamble to the user equipment (step S72). The random access preamble for the contention-free random access is selected by the target base station from a predetermined number (e.g. reserved 10) of sequences among 64 orthogonal sequences, for example. - Next, the user equipment transmits the random access preamble to the target base station (step S74). In this case, because there is no case where the same random access preamble is used by different user equipments at the same timing with the same frequency, a collision of random access preambles does not occur.
- Then, the target base station which has received the random access preamble determines a timing adjustment value for each user equipment according to the reception timing (step S76). Further, the target base station performs scheduling for a connection request of the user equipment. Note that, when a communication resource for a connection request cannot be allocated due to the lack of available communication resources, a random access fails.
- Then, the target base station transmits a random access response to the user equipment (step S78). The random access response contains the timing adjustment value determined by the target base station and scheduling information for a connection request by the user equipment. Receiving the random access response, the user equipment adjusts the transmission timing of a signal to the target base station by using the timing adjustment value contained in the random access response (step S80). After that, the user equipment establishes a connection with the target base station in the same manner as the contention-based random access procedure.
- As is understood from the above description, in the random access procedure, a random access can fail due to the collision of signals or the shortage of communication resources. Then, a random access is retried after the lapse of a random backoff period. During this period, a communication by the user equipment delays. Further, the time needed for calculation of a timing adjustment value and adjustment of transmission timing is the overhead of a communication. The use of the contention-free random access procedure reduces the possibility of signal collision. However, because the number of random access preambles which are reserved for the contention-free random access is limited, the contention-free random access procedure is not always available in a handover procedure. Therefore, for the purpose of preventing the degradation of service quality, it is effective to perform a random access during a handover procedure in a radio communication involving the carrier aggregation more efficiency as in two embodiments of the present invention which are described in detail below.
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Fig. 4 is a schematic view showing an outline of aradio communication system 1 according to an embodiment of the present invention. Referring toFig. 4 , theradio communication system 1 includes auser equipment 100, abase station 200a and abase station 200b. It is assumed that thebase station 200a is a serving base station for theuser equipment 100. - The
user equipment 100 is located inside acell 202a where a radio communication service is provided by thebase station 200a. Theuser equipment 100 can perform a data communication with another user equipment (not shown) via thebase station 200a over a communication channel formed by aggregating a plurality of component carriers (i.e. by carrier aggregation). However, because the distance between theuser equipment 100 and thebase station 200a is not short, there is a possibility that a handover is required for theuser equipment 100. Further, theuser equipment 100 is located inside acell 202b where a radio communication service is provided by thebase station 200b. Therefore, thebase station 200b can be a candidate for a target base station for a handover of theuser equipment 100. - The
base station 200a can communicate with thebase station 200b through a backhaul link (e.g. X2 interface). Various kinds of messages in the handover procedure as described with reference toFig. 1 , scheduling information related to the user equipment belonging to each cell or the like, for example, can be transmitted and received between thebase station 200a and thebase station 200b. Further, thebase station 200a and thebase station 200b can communicate with the MME, which is an upper node, through S1 interface, for example. - It should be noted that, when there is no particular need to distinguish between the
base station 200a and thebase station 200b in the following description of the specification, they are collectively referred to as abase station 200 by omitting the alphabetical letter at the end of the reference symbol. The same applies to the other elements. - A first embodiment of the present invention for making a more efficient random access during a handover procedure in a radio communication involving the carrier aggregation is described hereinafter with reference to
Figs. 5 to 9B . -
Fig. 5 is a block diagram showing an example of a configuration of theuser equipment 100 according to the embodiment. Referring toFig. 5 , theuser equipment 100 includes aradio communication unit 110, asignal processing unit 150, acontrol unit 160, an adjustmentvalue storage unit 162, and ameasurement unit 170. - The
radio communication unit 110 performs a radio communication with thebase station 200 over a communication channel formed by aggregating a plurality of component carriers with use of the carrier aggregation technology. -
Fig. 6 is a block diagram showing an example of a more detailed configuration of theradio communication unit 110. Referring toFig. 6 , theradio communication unit 110 includes anantenna 112, aswitch 114, a low noise amplifier (LNA) 120, a plurality of down-converters 122a to 122c, a plurality offilters 124a to 124c, a plurality of analogue-to-digital converters (ADCs) 126a to 126c, ademodulation unit 128, amodulation unit 130, a plurality of digital-to-analogue converters (DACs) 132a to 132c, a plurality offilters 134a to 134c, a plurality of up-converters 136a to 136c, acombiner 138, and a power amplifier (PA) 140. - The
antenna 112 receives a radio signal transmitted from thebase station 200 and outputs the received signal to theLNA 120 through theswitch 114. TheLNA 120 amplifies the received signal. The down-converter 122a and thefilter 124a separate a baseband signal of the first component carrier (CC1) from the received signal amplified by theLNA 120. Then, the separated baseband signal is converted to a digital signal by theADC 126a and output to thedemodulation unit 128. Likewise, the down-converter 122b and thefilter 124b separate a baseband signal of the second component carrier (CC2) from the received signal amplified by theLNA 120. Then, the separated baseband signal is converted to a digital signal by theADC 126b and output to thedemodulation unit 128. Further, the down-converter 122c and thefilter 124c separate a baseband signal of the third component carrier (CC3) from the received signal amplified by theLNA 120. Then, the separated baseband signal is converted to a digital signal by theADC 126c and output to thedemodulation unit 128. After that, thedemodulation unit 128 generates a data signal by demodulating the baseband signals of the respective component carriers and outputs the data signal to thesignal processing unit 150. - Further, when a data signal is input from the
signal processing unit 150, themodulation unit 130 modulates the data signal and generates baseband signals of the respective component carriers. Among those baseband signals, the baseband signal of the first component carrier (CC1) is converted to an analog signal by theDAC 132a. Then, a frequency component corresponding to the first component carrier in a transmission signal is generated from the analog signal by thefilter 134a and the up-converter 136a. Likewise, the baseband signal of the second component carrier (CC2) is converted to an analog signal by theDAC 132b. Then, a frequency component corresponding to the second component carrier in the transmission signal is generated from the analog signal by thefilter 134b and the up-converter 136b. Further, the baseband signal of the third component carrier (CC3) is converted to an analog signal by theDAC 132c. Then, a frequency component corresponding to the third component carrier in the transmission signal is generated from the analog signal by thefilter 134c and the up-converter 136c. After that, the generated frequency components corresponding to the three component carriers are combined by thecombiner 138, and the transmission signal is formed. ThePA 140 amplifiers the transmission signal and outputs the transmission signal to theantenna 112 through theswitch 114. Then, theantenna 112 transmits the transmission signal as a radio signal to thebase station 200. - Although the case where the
radio communication unit 110 handles three component carriers is described inFig. 6 , the number of component carriers handled by theradio communication unit 110 may be two, or four or more. - Further, instead of processing the signals of the respective component carriers in the analog region as in the example of
Fig. 6 , theradio communication unit 110 may process the signals of the respective component carriers in the digital region. In the latter case, at the time of reception, a digital signal converted by one ADC is separated into the signals of the respective component carriers by a digital filter. Further, at the time of transmission, after digital signals of the respective component carriers are frequency-converted and combined, the signal is converted into an analog signal by one DAC. The load of the ADC and the DAC is generally smaller when processing the signals of the respective component carriers in the analog region. On the other hand, when processing the signals of the respective component carriers in the digital region, a sampling frequency for AD/DA conversion is higher, and the load of the ADC and the DAC can thereby increase. - Referring back to
Fig. 5 , an example of a configuration of theuser equipment 100 is further described below. - The
signal processing unit 150 performs signal processing such as deinterleaving, decoding or error correction on the demodulated data signal that is input from theradio communication unit 110. Then, thesignal processing unit 150 outputs the processed data signal to an upper layer. Further, thesignal processing unit 150 performs signal processing such as encoding or interleaving on the data signal that is input from the upper layer. Then, thesignal processing unit 150 outputs the processed data signal to theradio communication unit 110. - The
control unit 160 controls the overall functions of theuser equipment 100 by using a processing device such as a central processing unit (CPU) or a digital signal processor (DSP). For example, thecontrol unit 160 controls the timing of data communication by theradio communication unit 110 according to scheduling information that is received from thebase station 200 by theradio communication unit 110. At this time, thecontrol unit 160 adjusts the timing of data transmission from theradio communication unit 110 by using a timing adjustment value which is notified from thebase station 200 in a random access procedure. Further, thecontrol unit 160 controls themeasurement unit 170 to measure the channel quality by using a reference signal from thebase station 200, which is a serving base station, and transmits the channel quality report to thebase station 200 through theradio communication unit 110. Further, thecontrol unit 160 controls themeasurement unit 170 to execute measurement during the periods of measurement gaps which are allocated by thebase station 200. - Further, in this embodiment, when two or more component carriers are allocated by the target base station during a handover procedure, the
control unit 160 determines whether a distance in the frequency direction between the two or more component carriers is smaller than a specific threshold. When the distance in the frequency direction is smaller than a specific threshold, thecontrol unit 160 skips a random access to the target base station for at least one of the two or more component carriers. Specifically, when first and second component carriers exist in close proximity in the frequency direction, thecontrol unit 160 makes a random access for the first component carrier and skips a random access for the second component carrier. The specific threshold may be the maximum value of a difference between two frequencies (e.g. the center frequencies of the respective component carriers) whose transmission timing can be adjusted appropriately based on one timing adjustment value notified from the target base station in a random access procedure. Thus, even when a random access is skipped for any of the component carriers, theuser equipment 100 can carry out a handover by appropriately adjusting the transmission timing for the component carrier. A transmission timing difference depends on a distance between each user equipment and a base station. Thus, a timing adjustment is typically used between user equipment and a base station for purposes of synchronization. When the user equipment handles a plurality of component carriers, it is also typical for the user equipment to adjust transmission timing for each component carrier because the timing difference depends on frequency. Generally, propagation characteristics include, but not limited to, characteristics of reflection, diffraction, scattering and propagation. These characteristics are known to depend on frequency of the component carrier. Therefore, if one frequency is close another frequency, the characteristic such as propagation delay is similar to each other between two different frequencies. These propagation characteristics affect the propagation delay amount of the signals. Therefore, in the case where the a distance in the frequency direction between the two component carriers is smaller than a specific threshold, propagation delay of the signals over the two component carriers is close to each other. Therefore, it becomes easier to adjust transmission timing of one component carrier based on the transmission timing of another component carrier. - Further, in the case where a new communication channel with a target base station is composed of three or more component carriers, when a distance in the frequency direction between certain component carriers among them is smaller than a specific threshold, the
control unit 160 may skip a random access for at least one of the component carriers. For example, it is assumed that a distance in the frequency direction between the component carrier CC1 and the component carrier CC2 among the component carriers CC1 to CC3 is smaller than a specific threshold, and a distance between the component carrier CC1, CC2 and the component carrier CC3 is larger than the specific threshold. In this case, thecontrol unit 160 may make a random access to the target base station for the component carriers CC1 and CC2 in one time and further make a random access to the target base station for the component carrier CC3. - In the case of skipping a random access for any of the component carriers, the
control unit 160 first executes a random access to the target base station for another component carrier which exists in close proximity to the relevant component carrier in the frequency direction. Then, thecontrol unit 160 stores the timing adjustment value which is notified from the target base station in the wake of the random access into the adjustmentvalue storage unit 162. After that, thecontrol unit 160 adjusts the transmission timing for the component carrier for which a random access is skipped by using the timing adjustment value stored in the adjustmentvalue storage unit 162. Then, thecontrol unit 160 makes an access to the target base station over the relevant component carrier at the adjusted transmission timing (the access in this case is not a random access). - The adjustment
value storage unit 162 stores a timing adjustment value for one or more component carriers which is input from thecontrol unit 160 by using a storage medium such as a hard disk or semiconductor memory. Then, in response to a command from thecontrol unit 160, the adjustmentvalue storage unit 162 outputs the stored timing adjustment value for use in adjustment of the transmission timing for another component carrier. - The
measurement unit 170 measures the channel quality for each of the component carriers by using a reference signal from thebase station 200 according to control from thecontrol unit 160, for example. Further, themeasurement unit 170 executes measurement for a handover with respect to each of the component carriers by using the measurement gaps which are allocated by thebase station 200. A result of the measurement executed by themeasurement unit 170 is converted to a predetermined format for a measurement report by thecontrol unit 160 and transmitted to thebase station 200 through theradio communication unit 110. After that, thebase station 200 determines, based on the measurement report, whether a handover should be executed or not for theuser equipment 100. -
Fig. 7 is a block diagram showing an example of a configuration of thebase station 200 according to the embodiment. Referring toFig. 7 , thebase station 200 includes aradio communication unit 210, aninterface unit 250, a component carrier (CC)management unit 260, and acontrol unit 280. - A specific configuration of the
radio communication unit 210 may be similar to the configuration of theradio communication unit 110 of theuser equipment 100 which is described above with reference toFig. 6 , although the number of component carriers to be supported, the requirements of processing performance or the like are different. Theradio communication unit 210 performs a radio communication with the user equipment over a communication channel which is formed by aggregating a plurality of component carriers with use of the carrier aggregation technology. - The
interface unit 250 mediates a communication between theradio communication unit 210 or thecontrol unit 280 and an upper node through the S1 interface illustrated inFig. 4 , for example. Further, theinterface unit 250 mediates a communication between theradio communication unit 210 or thecontrol unit 280 and another base station through the X2 interface illustrated inFig. 4 , for example. - The
CC management unit 260 holds data that indicates which component carrier each user equipment is using for communication with respect to each of the user equipments belonging to the cell of thebase station 200. Such data can be updated by thecontrol unit 280 when an additional user equipment joins the cell of thebase station 200 or when the existing user equipment changes its component carriers. Thus, thecontrol unit 280 can recognize which component carrier theuser equipment 100 is using by referring to the data held by theCC management unit 260. - The
control unit 280 controls the overall functions of thebase station 200 by using a processing device such as a CPU or a DSP. For example, thecontrol unit 280 executes an allocation of communication resources for data communication, i.e. scheduling, for theuser equipment 100 and other user equipments. Then, thecontrol unit 280 delivers scheduling information over a broadcast channel in a given sub-frame. Further, when thebase station 200 is a target base station of a handover of a radio communication involving the carrier aggregation, thecontrol unit 280 allocates two or more component carriers to a new communication channel. At this time, when two or more component carriers which are adjacent in the frequency direction are allocable, thecontrol unit 280 allocates the two or more component carriers to the new communication channel. Further, when two or more component carriers whose distance in the frequency direction is smaller than the above-described specific threshold are allocable, thecontrol unit 280 allocates the two or more component carriers to the new communication channel. -
Fig. 8 is an explanatory view to describe an allocation of component carriers by a target base station according to the embodiment.Fig. 8 shows a step-by-step handover including four stages in the case where a communication channel is composed of three component carriers CC1 to CC3. The first stage is before a handover. The second stage is after a handover of the component carrier CC1. The third stage is after a handover of the component carriers CC1 and CC2. The fourth stage is after a handover of all the component carriers. - First, in the first stage, the component carriers CC1 to CC3 are connected with a source base station. The positions of the component carriers CC1 to CC3 in the frequency direction may be any positions.
- Next, when a handover is completed for the component carrier CC1, the handover procedure proceeds to the second stage. The frequency band to which the component carrier CC1 is allocated in the target base station may be the same frequency band as the frequency band in the source base station or a different frequency band from the frequency band in the source base station.
- Then, the
control unit 280 of the target base station which has received a handover request for the component carrier CC2 determines whether the component carriers CC1 and CC2 can be allocated so that they are adjacent in the frequency direction. It is assumed in the example ofFig. 8 that the component carriers CC1 and CC2 can be allocated so that they are adjacent in the frequency direction. Thus, the control unit 28 allocates the component carriers CC1 and CC2 so that they are adjacent in the frequency direction (third stage). - Further, the
control unit 280 of the target base station which has received a handover request for the component carrier CC3 determines whether the component carrier CC1 or CC2 and the component carrier CC3 can be allocated so that they are adjacent in the frequency direction. It is assumed in the example ofFig. 8 that those component carriers are not allocable to be adjacent in the frequency direction. Then, the control unit 28 determines whether the component carrier CC3 can be allocated to a new communication channel so that a distance from the component carrier CC1 or CC2 in the frequency direction is smaller than a threshold FTH. The threshold FTH indicates the above-described specified threshold which can correspond to the maximum value of a difference between two frequencies whose transmission timing can be adjusted appropriately based on one timing adjustment value. It is assumed in the example ofFig. 8 that the component carrier CC3 can be allocated so that the distances from the component carriers CC1 and CC2 are smaller than the threshold FTH. Thus, thecontrol unit 280 allocates the component carrier CC3 to the position where the distances from the component carriers CC1 and CC2 are smaller than the threshold FTH (fourth stage). - It should be noted that,
Fig. 8 shows the case where, after a handover for one component carrier is completed, another component carrier is allocated to the new communication channel. However, the allocation of another component carrier to a new communication channel may be performed before completion of a handover for one component carrier. Further, in the example ofFig. 8 , it is described that, for a certain component carrier, the frequency band of the component carrier is changed at the time of an access to the target base station by theuser equipment 100. However, for a certain component carrier, a base station to which the component carrier is connected may be changed from the source base station to the target base station after the frequency band has changed in the source base station according to the above-described conditions related to the position in the frequency direction. - In addition to such an allocation of component carriers, the
control unit 280 controls thebase station 200 to operate in the same manner as the source base station or the target base station in the handover procedure which is descried with reference toFig. 1 . - A flow of a handover procedure according to the embodiment is described hereinafter with reference to
Figs. 9A and9B . Note that, in the following scenario, it is assumed that a handover procedure is performed among theuser equipment 100, thebase station 200a serving as a source base station, and thebase station 200b serving as a target base station. Further, for simplification of description, it is assumed in this scenario that theuser equipment 100 performs a radio communication by using two component carriers. Furthermore, for the procedure up to measurement in the user equipment (steps S2 to S14) in the typical handover procedure illustrated inFig. 1 , explanation is omitted because there is no significant difference. - Referring to
Fig. 9A , theuser equipment 100 first transmits a measurement report for the component carrier CC1, for example, to thebase station 200a (step S122). Receiving the measurement report, thebase station 200a determines the necessity of a handover based on the measurement report. For example, when a channel quality between theuser equipment 100 and thebase station 200b is better than a channel quality between theuser equipment 100 and thebase station 200a by a predetermined threshold or greater, it can be determined that a handover is necessary. In this case, thebase station 200a transmits a handover request message for the component carrier CC1 to thebase station 200b (step S124). Receiving the handover request message, thebase station 200b allocates the component carrier CC1 to any frequency band for a new communication channel with theuser equipment 100 according to the availability of a communication service. Then, thebase station 200b transmits a handover request confirm message to thebase station 200a (step S126). Receiving the handover request confirm message, thebase station 200a transmits a handover command for the component carrier CC1 to the user equipment 100 (step S128). - Receiving the handover command, the
user equipment 100 first acquires synchronization with the downlink channel of the component carrier CC1 of thebase station 200b (step S132). Then, theuser equipment 100 makes a random access to thebase station 200b by using a random access channel of the component carrier CC1 (step S134). During this period, thecontrol unit 160 of theuser equipment 100 stores the timing adjustment value for the component carrier CC1 which is notified from thebase station 200b into the adjustmentvalue storage unit 162. During the downlink synchronization and the random access, thebase station 200a forwards data addressed to theuser equipment 100 to thebase station 200b (step S136). - Then, after success in the random access for the component carrier CC1, the
user equipment 100 transmits a handover complete message for the component carrier CC1 to thebase station 200b (step S142). Receiving the handover complete message, thebase station 200b requests the MME to perform route update for the component carrier CC1 of the user equipment 100 (step S144). Upon updating the route of user data by the MME, theuser equipment 100 becomes able to communicate with another device through a new base station (i.e. thebase station 200b). Note that the request for route update may be performed with respect to each component carrier, or performed only once through a plurality of component carriers. Then, thebase station 200b transmits acknowledgement for the handover complete message to the user equipment 100 (step S146). - Further, referring to
Fig. 9B , theuser equipment 100 transmits a measurement report for the component carrier CC2 to thebase station 200a (step S152). Receiving the measurement report, thebase station 200a transmits a handover request message for the component carrier CC2 to thebase station 200b (step S154). - Receiving the handover request message, the
base station 200b allocates the component carrier CC2 to any frequency band for the new communication channel with theuser equipment 100 according to the availability of a communication service. In this scenario, it is assumed that the frequency band to which the component carrier CC2 is allocated is the frequency band which is adjacent to the component carrier CC1 or the frequency band in which the distance from the component carrier CC1 in the frequency direction is smaller than the threshold FTH. After that, thebase station 200b transmits a handover request confirm message to thebase station 200a (step S156). Receiving the handover request confirm message, thebase station 200a transmits a handover command for the component carrier CC2 to the user equipment 100 (step S158). - Then, in the
user equipment 100 which has received the handover command, thecontrol unit 160 reads the timing adjustment value for the component carrier CC1 which is stored in the adjustmentvalue storage unit 162. Thecontrol unit 160 then adjusts the access timing to thebase station 200b for the component carrier CC2 by using the timing adjustment value which is read from the adjustment value storage unit 162 (step S164). During this period, thebase station 200a forwards data addressed to theuser equipment 100 to thebase station 200b (step S166). - The
user equipment 100 then transmits a handover complete message for the component carrier CC2 to thebase station 200b (step S172). Receiving the handover complete message, thebase station 200b requests the MME to perform route update for the component carrier CC2 of the user equipment 100 (step S174). Then, thebase station 200b transmits acknowledgement for the handover complete message to the user equipment 100 (step S176). - The first embodiment of the present invention is described above with reference to
Figs. 5 to 9B . According to the embodiment, in a radio communication involving the carrier aggregation, when a distance in the frequency direction between two or more component carriers which are allocated to a communication channel with a target base station is smaller than a specific threshold, theuser equipment 100 skips a random access to the target base station for at least one of the two or more component carriers. Therefore, an accumulation of delays corresponding to the number of component carriers due to a failure of a random access does not occur, and the degradation of service quality caused by a random access during a handover procedure is suppressed. Further, theuser equipment 100 adjusts the transmission timing for the component carrier for which a random access has been skipped by using the timing adjustment value notified from the target base station for the component carrier for which a random access has been made. A communication can be thereby performed at appropriate timing also in the component carrier for which a random access has been skipped. Further, because it is not necessary in the target base station to redundantly determine the timing adjustment value for the component carrier, the overhead of a communication is reduced. - Further, according to the embodiment, when two or more component carriers which are adjacent in the frequency direction are allocable, the
base station 200 serving as a target base station allocates the two or more component carriers to a new communication channel with theuser equipment 100. Further, when two or more component carriers whose distance in the frequency direction is smaller than a specific threshold are allocable, thebase station 200 allocates the two or more component carriers to a new communication channel. The opportunity of skipping a random access by theuser equipment 100 during a handover procedure thereby increases, and it is possible to more effectively obtain the above-described advantages such as prevention of an accumulation of delays due to a failure of a random access or reduction of overhead. Note that the positions in the frequency direction of those component carriers may be varied after the end of a series of handover procedure as a matter of course. - A second embodiment of the present invention for making a more efficient random access during a handover procedure in a radio communication involving the carrier aggregation is described hereinafter with reference to
Figs. 10 to 12B . -
Fig. 10 is a block diagram showing an example of a configuration of theuser equipment 300 according to the embodiment. Referring toFig. 10 , theuser equipment 300 includes aradio communication unit 110, asignal processing unit 150, acontrol unit 360, and ameasurement unit 170. - The
control unit 360 controls the overall functions of theuser equipment 300 by using a processing device such as a CPU or a DSP. For example, thecontrol unit 360 controls the timing of data communication by theradio communication unit 110 according to scheduling information that is received from thebase station 400 by theradio communication unit 110. Further, thecontrol unit 360 controls themeasurement unit 170 to measure the channel quality by using a reference signal from thebase station 400, which is a serving base station, and transmits the channel quality report to thebase station 400 through theradio communication unit 110. Further, thecontrol unit 360 controls themeasurement unit 170 to execute measurement during the periods of measurement gaps which are allocated by thebase station 400. - Further, in this embodiment, when one or more component carriers are allocated to a new communication channel by the
base station 400 during a handover procedure, thecontrol unit 360 controls theradio communication unit 110 to make a random access to thebase station 400. By a random access for the one or more component carriers, a new communication channel with thebase station 400 is established. Then, thecontrol unit 360 controls theradio communication unit 110 to notify thebase station 400 of the timing of a random access for one or more other (remaining) component carriers to constitute the communication channel over the established new communication channel. Thebase station 400 as a target base station can thereby reserve communication resources necessary in the random access procedure by theuser equipment 300 in advance based on the notified timing. -
Fig. 11 is a block diagram showing an example of a configuration of thebase station 400 according to the embodiment. Referring toFig. 11 , thebase station 400 includes aradio communication unit 210, aninterface unit 250, aCC management unit 260, and acontrol unit 480. - The
control unit 480 controls the overall functions of thebase station 400 by using a processing device such as a CPU or a DSP. For example, thecontrol unit 480 executes an allocation of communication resources for data communication for theuser equipment 300 and other user equipments. Then, thecontrol unit 480 delivers scheduling information over a broadcast channel in a given sub-frame. Further, when thebase station 400 is a target base station of a handover by theuser equipment 300, thecontrol unit 480 sequentially allocates two or more component carriers to a new communication channel with theuser equipment 300. At this time, over the new communication channel which is established by a random access for one or more component carriers, the timing of a random access for one or more other component carriers to constitute the communication channel can be notified from theuser equipment 300 as described above. Receiving such a notification, thecontrol unit 480 reserves communication resources for theuser equipment 300 according to the notified timing so that the subsequent random access for the one or more other component carriers succeeds. The communication resources reserved thereby include resources of an uplink for a connection request from theuser equipment 300, resources of a downlink for acknowledgement to the connection request or the like. - In addition to such a reservation of communication resources, the
control unit 480 controls thebase station 400 to operate in the same manner as the source base station or the target base station in the handover procedure which is descried with reference toFig. 1 . - A flow of a handover procedure according to the embodiment is described hereinafter with reference to
Figs. 12A and12B . Note that, in the following scenario, it is assumed that a handover procedure is performed among theuser equipment 300, thebase station 400a serving as a source base station, and thebase station 400b serving as a target base station. Further, likeFigs. 9A and9B , for simplification of description, it is assumed in this scenario that theuser equipment 300 performs a radio communication by using two component carriers. Furthermore, for the procedure up to measurement in the user equipment (steps S2 to S14) in the typical handover procedure illustrated inFig. 1 , explanation is omitted because there is no significant difference. - Referring to
Fig. 12A , theuser equipment 300 first transmits a measurement report for the component carrier CC1, for example, to thebase station 400a (step S222). Receiving the measurement report, when thebase station 400a determines that a handover is necessary, thebase station 400a transmits a handover request message for the component carrier CC1 to thebase station 400b (step S224). Receiving the handover request message, thebase station 400b allocates the component carrier CC1 to any frequency band for a new communication channel with theuser equipment 300 according to the availability of a communication service. Then, thebase station 400b transmits a handover request confirm message to thebase station 400a (step S226). Receiving the handover request confirm message, thebase station 400a transmits a handover command for the component carrier CC1 to the user equipment 300 (step S228). - Receiving the handover command, the
user equipment 300 first acquires synchronization with the downlink channel of the component carrier CC1 of thebase station 400b (step S232). Then, theuser equipment 300 makes a random access to thebase station 400b by using a random access channel of the component carrier CC1 (step S234). When the random access is success, a new communication channel between theuser equipment 300 and thebase station 400b is established. During the downlink synchronization and the random access, thebase station 400a forwards data addressed to theuser equipment 300 to thebase station 400b (step S236). - Then, after success in the random access for the component carrier CC1, the
user equipment 300 transmits a handover complete message for the component carrier CC1 to thebase station 400b (step S242). Receiving the handover complete message, thebase station 400b requests the MME to perform route update for the component carrier CC1 of the user equipment 300 (step S244). Then, thebase station 400b transmits acknowledgement for the handover complete message to the user equipment 300 (step S246). - Further, referring to
Fig. 12B , theuser equipment 300 transmits a measurement report for the component carrier CC2 to thebase station 400a (step S252). Receiving the measurement report, thebase station 400a transmits a handover request message for the component carrier CC2 to thebase station 400b (step S254). - Receiving the handover request message, the
base station 400b allocates the component carrier CC2 to any frequency band for the new communication channel with theuser equipment 300 according to the availability of a communication service. The frequency band to which the component carrier CC2 is allocated may be any frequency band. After that, thebase station 400b transmits a handover request confirm message to thebase station 400a (step S256). Receiving the handover request confirm message, thebase station 400a transmits a handover command for the component carrier CC2 to the user equipment 300 (step S258). - Then, the
user equipment 300 notifies the timing of a random access (RA) for the component carrier CC2 to thebase station 400b over the established new communication channel (which has the component carrier CC1) (step S260). Thebase station 400b then reserves communication resources according to the notified timing so that a random access for the component carrier CC2 by theuser equipment 300 does not fail due to the shortage of communication resources (step S261). - After that, the
user equipment 300 acquires synchronization with the downlink channel of the component carrier CC2 of thebase station 400b (step S262). Then, theuser equipment 300 makes a random access to thebase station 400b by using a random access channel of the component carrier CC2 at the timing notified to thebase station 400b in the step S260 (step S264). Such a random access is likely to succeed unless signal collision occurs because communication resources for a connection request from theuser equipment 300 and acknowledgement for the request are reserved by thebase station 400b. During the downlink synchronization and the random access, thebase station 400a forwards data addressed to theuser equipment 300 to thebase station 400b (step S266). - The
user equipment 300 then transmits a handover complete message for the component carrier CC2 to thebase station 400b (step S272). Receiving the handover complete message, thebase station 400b requests the MME to perform route update for the component carrier CC2 of the user equipment 300 (step S274). Then, thebase station 400b transmits acknowledgement for the handover complete message to the user equipment 300 (step S276). - The second embodiment of the present invention is described above with reference to
Figs. 10 to 12B . According to the embodiment, at the time of a handover in a radio communication involving the carrier aggregation, the timing of a random access for a component carrier for which a handover has not been completed is notified from theuser equipment 300 to thebase station 400 over a component carrier for which a handover has been completed. Thebase station 400 can thereby reserve communication resources so that the subsequent random access for another component carrier from theuser equipment 300 does not fail due to the shortage of communication resources. As a result, the possibility of a failure of a random access is reduced, and the degradation of service quality due to an accumulation of delays is suppressed. -
- 1
- RADIO COMMUNICATION SYSTEM
- 100, 300
- USER EQUIPMENT
- 110
- RADIO COMMUNICATION UNIT (USER EQUIPMENT)
- 160, 360
- CONTROL UNIT (USER EQUIPMENT)
- 200, 400
- BASE STATION
- 210
- RADIO COMMUNICATION UNIT (BASE STATION)
- 280, 480
- CONTROL UNIT (BASE STATION)
Claims (6)
- A target base station of a handover procedure performed by a user equipment (100) from a source base station (200a) to the target base station (200b), the target base station (200b) comprising:a control unit (160) configured to allocate three or more component carriers for the user equipment (110) during the handover procedure, wherein a first component carrier (CC1) of the three or more component carriers corresponds with a first frequency band, and a second component carrier (CC2) of the three or more component carriers corresponds with a second frequency band;a radio communication unit (110) configured to:receive a random access preamble from the user equipment (110) on the first component carrier (CC1) among the three or more component carriers;transmit, to the user equipment (110) on the first component carrier (CC1), a random access response including a uplink timing adjustment value;receive, from the user equipment (110) on the first component carrier (CC1) after the handover procedure, a first uplink transmission adjusted based on the uplink timing adjustment value;on condition that a frequency distance between the first frequency band and the second frequency band is smaller than a predetermined threshold, receive, from the user equipment (110) on the second component carrier (CC2) after the handover procedure, a second uplink transmission adjusted based on the uplink timing adjustment value, wherein the a random access procedure on the second component carrier (CC2) is skipped during the handover procedure, oron condition that the frequency distance between the first frequency band and the second frequency band is larger than the predetermined threshold, perform the random access procedure on the second component carrier (CC2) with user equipment (110),wherein the transmission timing of the first uplink transmission on the first frequency band and the transmission timing of the second uplink transmission on the second frequency band are adjusted based on the one uplink timing adjustment value,wherein a first bandwidth used for the first uplink transmission on the first frequency band is 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, or 20 MHz, wherein a second bandwidth used for the second uplink transmission on the second frequency band is 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, or 20 MHz.
- The target base station according to claim 1, wherein the first bandwidth used for the first uplink transmission on the first frequency band is 20 MHz, wherein the second bandwidth used for the second uplink transmission on the second frequency band is 10 MHz or 20 MHz.
- The target base station according to claim 1, wherein the predetermined threshold indicates a maximum frequency distance between two frequency bands whose two transmission timings can be adjusted appropriately based on one timing adjustment value.
- A method for a target base station of a handover procedure performed by a user equipment (110) from a source base station (200a) to the target base station (200b), the method comprising:allocating three or more component carriers for the user equipment (110) during the handover procedure, wherein a first component carrier (CC1) of the three or more component carriers corresponds with a first frequency band, and a second component carrier (CC2) of the three or more component carriers corresponds with a second frequency band;receiving, via a radio transceiver, a random access preamble from the user equipment (110) on the first component carrier (CC1) among the three or more component carriers;transmitting, to the user equipment (110) on the first component carrier (CC1) via the radio transceiver, a random access response including a uplink timing adjustment value;receiving, from the user equipment (110) on the first component carrier (CC1) via the radio transceiver after the handover procedure, a first uplink transmission adjusted based on the uplink timing adjustment value;on condition that a frequency distance between the first frequency band and the second frequency band is smaller than a predetermined threshold, receiving, from the user equipment (110) on the second component carrier (CC2) via the radio transceiver after the handover procedure, a second uplink transmission adjusted based on the uplink timing adjustment value, wherein a random access procedure on the second component carrier (CC2) is skipped during the handover procedure; oron condition that the frequency distance between the first frequency band and the second frequency band is larger than the predetermined threshold, performing the random access procedure on the second component carrier (CC2) with user equipment (110),wherein the transmission timing of the first uplink transmission on the first frequency band and the transmission timing of the second uplink transmission on the second frequency band is adjusted based on the one uplink timing adjustment value,wherein a first bandwidth used for the first uplink transmission on the first frequency band is 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, or 20 MHz, wherein a second bandwidth used for the second uplink transmission on the second frequency band is 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, or 20 MHz.
- The method according to claim 4, wherein the first bandwidth used for the first uplink transmission on the first frequency band is 20 MHz, wherein the second bandwidth used for the second uplink transmission on the second frequency band is 10 MHz or 20 MHz.
- The method according to claim 4, wherein the predetermined threshold indicates a maximum frequency distance between two frequency bands whose two transmission timings can be adjusted appropriately based on one timing adjustment value.
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JP2009285374A JP5454123B2 (en) | 2009-12-16 | 2009-12-16 | Method for handover, terminal device and radio communication system |
PCT/JP2010/007037 WO2011074201A1 (en) | 2009-12-16 | 2010-12-02 | Method for performing handover, user equipment, and radio communication system |
EP10837238.4A EP2514250B1 (en) | 2009-12-16 | 2010-12-02 | User equipment, base station and radio communication system for performing handover |
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EP10837238.4A Division EP2514250B1 (en) | 2009-12-16 | 2010-12-02 | User equipment, base station and radio communication system for performing handover |
EP10837238.4A Division-Into EP2514250B1 (en) | 2009-12-16 | 2010-12-02 | User equipment, base station and radio communication system for performing handover |
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EP3589063B1 true EP3589063B1 (en) | 2021-10-06 |
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EP10837238.4A Active EP2514250B1 (en) | 2009-12-16 | 2010-12-02 | User equipment, base station and radio communication system for performing handover |
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IN (1) | IN2012DN05083A (en) |
RU (2) | RU2557558C2 (en) |
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US20120243510A1 (en) | 2012-09-27 |
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US20210274408A1 (en) | 2021-09-02 |
RU2015123434A (en) | 2015-12-10 |
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